This invention pertains to fiber communication systems and more particularly to the transmission and routing of several multiplexed wavelengths on fiber carrying light simultaneously in two directions of propagation.
Fiber optic networks include a variety of optical components such as transmitters, receivers, optical amplifiers and Wavelength Division Mulitplexers (WDMs). The amount of data that can be transmitted by these networks depends on the equipment bandwidth and the optimization of all the parts to support this bandwidth. Recently, service providers have had to double their system""s capacity approximately every three years due to the increases in communication traffic. A common method of increasing network capacity and system bandwidth is to upgrade the Time Division Multiplexed (TDM) equipment along the transmission path. For example, 10 Gbit/sec systems are being installed today for high-speed networks, replacing slower speed channels with data rates such as 622 Mbit/sec. Another cost effective means for increasing system bandwidth is to install equipment for wavelength division multiplexing (WDM).
In a WDM system, multiple wavelengths ki (where xe2x80x9cixe2x80x9d is an integer index) are transmitted on the same fiber, and each wavelength is used as an optical carrier for a TDM channel, such as a 2.5 or 10 Gbit/sec channel. All channels pass through similar optical components, such as fibers, WDMs and inline amplifiers. One important parameter in optical networks is the optical gain and loss experienced by individual channels as they propagate from the transmitting to the receiving station. For amplifiers, this parameter is termed Amplifier Gain Flatness. An asymmetry in net gain or loss between these channels leads to a reduction in overall system performance. Hence it is important to equalize and reduce the total loss of these channels as well as reduce the asymmetry in overall amplifier gain when all channels are present simultaneously, including when certain channels arbitrarily disappear due to equipment failure.
Channel routing is often performed in WDM networks. A common routing method shown in FIG. 1A employs three-port WDM elements, each with three ports P1, P2, P3 and an internal filter F1. These elements are available commercially from companies such as E-Tek of San Jose, Calif. and DiCon Fiberoptics of Berkley, Calif. If more than one channel is to be routed, then several of these elements are cascaded serially. However, channels that are routed first pass through fewer components and hence have less loss than those routed later. Hence, channel losses are asymmetric, when reduces overall system performance, particularly if channels are added later to the network. FIG. 1B shows how several three-port WDM elements are integrated into a single multi-channel element for routing four wavelengths k1, k2, k3, k4 through six pots P1-P6. Such elements have the same disadvantages as the three-port element of FIG. 1A. Multi-channel elements of this type are available from companies such as Optical Corporation of America of Marlborough, Mass.
It is generally accepted that network configurations that allow bi-directional transmission on the same fiber offer several advantages. Referring to FIG. 2, there is shown a prior art, bi-directional amplifier system, disclosed in U.S. Pat. No. 5,604,627 (Kohn), that employs two three-port WDM elements. In this system, wavelength bands k1 and k2 are defined for each direction of propagation and the three-port WDM elements WDMi are configured to deflect a respective wavelength band ki from a common input port to the appropriate optical amplifier Ai. The disadvantage of this model is that it requires the use of two amplifiers and two WDM elements for bi-directional operation on the same fiber, which in-turn increases system complexity and cost.
Referring to FIG. 3, there is shown a less-costly system configuration that is employed by BOSCH in its ONS-100 Optical Networking System. In this system, one amplifier and four three-port WDM elements are used in such a manner that the optical signal is transmitted through the amplifier in the same direction, regardless of the direction of transmission in the network, i.e. east to west or west to east. This configuration allows for bi-directional propagation on the same fiber using only one amplifier. However, the BOSCH system also increases the number of required WDM elements from two to four. The elimination of an expensive amplifier element reduces system cost, but doubling the number of WDM elements pushes system cost back up again.
Advantage of bi-directional amplifiers and a specific embodiment of a bi-directional amplifier configuration are also disclosed in U.S. Pat. No. 5,633,741 (Giles). Giles teaches the use of two four-port optical circulators C1 and C2 in conjunction with fiber gratings G1 and G2 and amplifiers A1 and A2 to achieve a bi-directional amplifier system as shown in FIG. 4. The disadvantage in such a method is that optical circulators, which are commercially available from companies such as JDS Fitel of Nepean, Canada, and The Kaifa Group of Sunnyvale, Calif., are complex and costly optical elements. The complexity of optical circulators and fiber gratings, in addition to the doubling in required number of components for bi-directional operation leads to an increase in overall system complexity and cost. Fiber gratings are specialized optical components and are commercially available from companies such as Lucent Technologies of Allentown, Pa.
The drawback of the devices shown in the prior art is their failure to provide a low-cost system (e.g., by minimizing the number of parts) or to enable the control of multi-channel gain and loss equalization for bi-directional operation on the same fiber. The objectives of this invention are therefore: (1) to reduce overall system cost by reducing the number of deployed amplifier and WDM elements and simplifying their manufacturability, (2) to equalize the losses incurred by all channels using alternate configurations, regardless of which channels are added or changed in the future, and (3) to provide a provision to flatten net amplifier gain for all wavelengths by incorporating other optical elements such as attenuators. It is a further objective of this invention to remedy the general drawbacks described in the background section.
In summary, the present invention is a four-port WDM element and a number of variations thereof that can be used as a principal component of a bidirectional optical amplifier.
In particular, the four-port WDM elements of the present invention can be used to add/remove any arbitrary channel or number of channels to/from a WDM communications network while maintaining similar insertion losses for all propagating channels. In one embodiment, attenuators and/of filters are employed by the WDM element to flatten the gain profile of any amplifier in which the WDM is incorporated. A four-port WDM element can be combined with a single amplifier element to form a bi-directional optical amplifier. Advantages of such an optical amplifier include: reduced cost due to simplified design, equalized losses of all channels, and flattened amplifier gain across all wavelengths.
An internal configuration of a basic embodiment of a four-port WDM element is shown in FIG. 5A. This WDM element resembles an xe2x80x9cXxe2x80x9d, where each optical port Pi lies along a respective arm of the xe2x80x9cXxe2x80x9d and a filter F is inserted at the intersection of the arms. Light launched into a first port (e.g., P1) is directed to the filter F, which selectively reflects certain channels or wavelengths to a receiving second port (e.g., P2). A third port (e.g., P3) receives the channels in the light launched into the first port P1 that are transmitted by the filter F. Similarly, light that is launched into the third port P3 is partially transmitted to the first port P1 by the filter F. The other channels in the light launched into the third port P3 are reflected by the filter F and are collected by a fourth port (e.g., P4). The resulting four-port WDM is symmetrical, meaning that light launched into the second and fourth ports behaves analogously to light launched into the first and third ports, respectively.
The present invention also includes various embodiments of bi-directional optical amplifiers that can be formed using the WDM elements of the present invention. In a basic bi-directional optical amplifier embodiment, shown in FIG. 7, the second and third ports of the WDM are coupled to an amplifier element A1. The ports P1 and P4 receive communication signals traveling in opposite directions. Each input signal can include multiple channels at different wavelengths. The filter characteristics are selected so the light input to the first port P1 is reflected to the second port P2 and then amplified by the amplifier element A1. The light input to the fourth port P4 is transmitted by the filter to the second port P2 and then passes through the amplifier A1 in the same direction as the light input to the first port P1. Thus, the four port WDM of the present invention enables the implementation of a bi-directional amplifier that requires only one WDM and one amplifier element.
Other disclosed embodiments of bi-directional amplifiers can be used to attenuate or add selected channels and amplify any number of channels.